Environmental Engineering Reference
In-Depth Information
Other Aerodynamic Analyses
While the Blade Element Momentum (BEM) approach has been a useful tool, the in-
ability of BEM to accurately treat design conditions such as operation at large yaw angles
and wind turbine response during short-term transients creates the need for a better wind
turbine analysis technique. Along with the search for a better analysis technique it should be
noted that all of potential improved techniques under consideration use a lifting line model in
which the flow conditions are evaluated along the axis of the blade (usually at 1/4 chord) and
these conditions are used to determine the angle of attack which in turn is used as an entry to
a table of two-dimensional airfoil characteristics. Neither the span-wise nor the chord-wise
variations in airfoil performance are modeled although Schreck
et al.
[2007] have examined
the influence of rotation effects on wind turbine airfoils.
The need for a more accurate wind turbine analysis technique has produced several
candidates that incorporate analysis of the wind turbine wake. There are two principle ap-
proaches, use of a
vortex wake
(either prescribed or free) and the
generalized dynamic wake
(GDW) method. Both of these methods claim to meet the challenge of modeling transient
HAWT behavior in a turbulent wind.
GDW Method
Currently, GDW analysis is being used to determine loads for wind turbine certification.
Adopted from a technique developed for helicopter analysis [Pitt and Peters 1981 and Peters
and He 1989], the GDW method is based on a potential flow solution of Kinner [1937]. Us-
ing the acceleration potential, Kinner obtained a general solution for flow about a circular
disk by considering the nonlinear convective flow acceleration to be based on the uniform
free-stream wind speed, thus linearizing the Euler Equations.
While Kinner used boundary conditions on the disk for an airfoil, the Pitt and Peters
solution used boundary conditions that allow flow through the disk such as occurs for a rotor.
When using the acceleration potential, the linearized Euler equations are transformed into a
LaPlace equation with the pressure as the dependent variable. This formulation of the flow
about a rotor has a cylindrical, skewed wake that accounts for the trailing wake and does not
require a tip-loss model. The GDW approach has thus far failed to give accurate results near
the tips of helicopter blades. However, because the wake from a wind turbine expands rather
than contracts (as is the case with a helicopter), there is no reversal of the induced velocity
near the tips of wind turbine blades. Thus, the use of the GDW approach could be more ac-
curate for wind turbines than for helicopters.
Advantages claimed for the GDW method include the ability to model transient aerody-
namics, tip losses and operation at large yaw angles. However, there are also disadvantages
associated with the GDW method when compared to strip theory. These are that the GDW
method
-
is poorest at wind speeds where induced velocities are large compared to the free
stream wind velocity
-
does not determine the induced rotational wind speed
-
does not allow for blade coning or the deflection of the blades
-
has so far failed to determine conditions near the blade tips
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